In ethology, a fixed action pattern (FAP) is an instinctivebehavioral sequence that is indivisible and runs to completion.[citation needed] Fixed action patterns are invariant and are produced by a neural network known as the innate releasing mechanism in response to an external sensory stimulus known as a sign stimulus or releaser (a signal from one individual to another).

Initially in classical ethology the behavioral sequence was thought to be inevitable triggered whenever the releaser was presented. However, more recent research [citation needed] suggests that the production of the behavior is less automatic and as a consequence the term has been replaced by species-specific or species typical behavior [1] London:Penguin.

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A mating dance may be used as an example. Many species of birds engage in a specific series of elaborate movements, usually by a brightly colored male. How well they perform the "dance" is then used by females of the species to judge their fitness as a potential mate. The key stimulus is typically the presence of the female.

Another example of fixed action patterns is the red-bellied stickleback (fish). The male turns a bright red/blue; colour during the breeding season. This colour change is the fixed action pattern in response to an increasing day length which is the sign stimulus. During this time they are also naturally aggressive towards other red-bellied sticklebacks, another FAP. However anything that is red, or has the appearance of being red, will bring about this FAP. The proximate response to this is that due to the stimuli, a nerve sends a signal to attack that red item. The ultimate cause of this behavior stems from the fact that the stickleback needs the area in which it is living for either habitat, food, mating with other sticklebacks, or other purposes. This interaction was studied by Niko Tinbergen.

Another well known case is the classic experiments by Tinbergen and Lorenz on the Graylag Goose. Like similar waterfowl, it will roll a displaced egg near its nest back to the others with its beak. The sight of the displaced egg triggers this mechanism. If the egg is taken away, the animal continues with the behavior, pulling its head back as if an imaginary egg is still being maneuvered by the underside of its beak. However, it will also attempt to move other egg shaped objects, such as a golf ball, door knob, or even an egg too large to have possibly been laid by the goose itself (a supernormal stimulus).[2]

Although fixed action patterns are most common in animals with simpler cognitive capabilities, humans also demonstrate fixed action patterns.[How to reference and link to summary or text] [reference: James E. Mazur's Learning and Behavior, Sixth Edition] For example, infants grasp strongly with their hands as a response to tactile stimulus. This is thought to be a vestigial mechanism where when threatened by a predator a young primate would grab on to a parent's fur so the parent could climb to safety without having to hold its child[How to reference and link to summary or text] (see also reflex action). Another FAP shared by some animals, including humans, is yawning, which often triggers yawning in other individuals. Yawns last around 6 seconds and are difficult to stop once initiated. Yawning, whether seen, heard or both, then serves as a releaser in nearby animals.[3]

Some species have evolved to exploit the fixed action patterns of other species by mimicry of their sign stimulus. Replicating the releasing mechanism required to trigger a FAP is known as code-breaking. A well known example of this is brood parasitism, where one species will lay its eggs in the nest of another species, which will then parent its young. A young North American cowbird, for example, provides a supernormal stimulus to its foster parent, which will cause it to forage rapidly in order to satisfy the larger bird's demands.[4] In a natural situation a nestling will provide higher levels of stimulus with noisier, more energetic behavior, communicating its urgent need for food. Parents in this situation should work extra hard to provide food, otherwise their own offspring are likely to die of starvation.